71: Thumb's the Word

Parents often try to figure out whether toddlers will be left- or right-handed. When they clasp their hands, which thumb is on top? If they grab a crayon, which hand do they use? Many children, under the influence of rapidly developing brains, flip-flop between right and left, so parents mistakenly think they must be ambidextrous. In July scientists came up with an easy—and early—way to tell: Look at the sonograms. Fetuses sucking their right thumb in the womb tend to be right-handed, and vice versa.

Still, the studies by psychologist Peter Hepper and his colleagues at Queen’s University in Belfast have not cleared up what determines righties and lefties in the first place. Most experts believe the dominant hand is a by-product of a genetically driven process of brain organization, which locates specific functions—like language—in one hemisphere of the brain or the other. But this research, argues Hepper, “shows handedness present before evidence of differences in structure or function of the brain hemispheres.” Therefore, he argues, hand preference causes brain lateralization—or there is no link between the two processes.

Hepper speculates that thumb preference in the womb—and by extension hand preference in life—is based on the side of the body that develops faster. The genetic model still holds, says Hepper, but he thinks it’s a matter of degree. “We need to determine just where genetic influence ends and environmental factors begin.”